1. Field of the Invention
The present invention relates to an anti-reflection film, to an organic EL (Electro Luminescence) device, and to a display medium that uses the anti-reflection film and the organic EL device. In particular, the present invention relates to an anti-reflection film that suppresses reflectance in three light emission peak wavelengths in a light source that has light emission peaks in the so-called primary colors, such as a three band fluorescent lamp. The present invention also relates to a three band organic EL device having light emission peaks in the primary colors, and to a display device such as a liquid crystal monitor, which uses the anti-reflection film and the three band organic EL device.
2. Description of the Related Art
There is a harmful effect in conventional optical components such as lenses made from optical glass having a relatively large index of refraction. If light incident to the optical component reflects off a surface of the optical component, transmittivity will drop by the amount of light reflected, and/or a projection into the surface due to the reflection will develop.
With eyeglasses, for example, the amount of light that passes through an eyeglass lens is further reduced in the evening, when the amount of light is insufficient, and it becomes very hard to see. Further, several lenses are combined together in a camera lens, and therefore the amount of light that passes through the camera lens is reduced by the amount of lenses combined, and a screen becomes darker. Furthermore, for cases of a display medium such as a television monitor, external light such as that from lighting and the like is reflected by a display screen, leading to projections and flaring, and markedly worsening visibility.
An anti-reflection film has generally been implemented in conventional optical components such as lenses in order to prevent such harmful effects due to reflection by these types of surfaces.
Single layer films such as magnesium fluoride (MgF2), for example, are used as the anti-reflection film. A thin film of the anti-reflection film is formed at a thickness of one-quarter of the wavelength of light on a surface of the lens. Light reflected from a front surface and light reflected from a rear surface will mutually cancel out due to interference, thus suppressing reflected light. However, the reflection of only one wavelength of light can be suppressed with a single-layer film, and in practice a multi-layer anti-reflection film that combines several layers is used.
The multi-layer anti-reflection film is, for example, a film in which multiple layers of substances having different indexes of refraction are evaporated on a glass substrate.
Specifically, magnesium fluoride may be used as the film material in order to suppress green light reflection, for example. The wavelength of green light is 550 nm, and the index of refraction of magnesium fluoride is 1.39. The wavelength within the film therefore becomes 396 nm, and the film may be formed having a thickness of one-quarter of this wavelength, which is 99 nm.
As described above, however, this film alone can only suppress the reflection of one wavelength of light (green light) at 550 nm. A multiple-layer film in which several layers overlap is therefore made using the same principle. The amount of reflection that is suppressed naturally increases as the number of layers increases, and the amount of light incident to an inside portion also becomes greater.
The anti-reflection film is normally designed to reduce the reflectance over the entire visible region.
The manner in which colors of objects are seen differs according to the type of light source present. For example, faces and clothing that can be seen naturally under sunlight take on an unnatural orange color under tunnel lighting, such as low voltage sodium lamps. That is, sunlight, under which the colors of objects can be seen naturally, is considered as “good lighting”. Tunnel lighting, under which the colors of objects are seen as unnatural, is considered as “poor lighting”.
The fact that colors of objects can be seen differently according to the type of light source present is referred to as “color rendering”, and this becomes one standard for determining lighting quality. The color rendering standard for lighting quality has been quantified as a color rendering index, and the color rendering index is formulated by the Commission Internationale de l'Eclairage (CIE) and by Japan Industrial Standards (JIS).
For example, it is said to be desirable to make the light emission peaks (light emission maxima) conform to the so-called primary colors in order to maximize the color reproducibility of a monitor, or the color rendering of a fluorescent light.
Note that the term “primary colors” as used here refers to colors having wavelengths in the vicinity of 450 nm, 540 nm, and 610 nm (for example, these values ±20 nm), colors to which the human eye is most sensitive. Specifically, this is discussed in: Thornton, W. A., “Matching Lights, Metamers, and Human Visual Response”, J. Color and Appearance, 2(1), pp. 23-9 (1973); Brill, M. H., Finlayson, D., Hubel, P. M., and Thornton, W. A., “Prime Colors and Color Imaging”, Proc. IS&T/SID 6th Color Imaging Conference, pp. 33-42 (1998); Finlayson, G. D., and Morovic, P. M., “Metamer Crossovers of Infinite Metamer Sets”, Proc. IS&T/SID 8th Color Imaging Conference, pp. 13-7 (2000), and the like.
Three band fluorescent lamps, for example, which are often used for lighting and as backlights of liquid crystal displays and the like, possess light emission peaks in three wavelengths, 435 nm, 545 nm, and 610 nm.
Further, using organic light emitting devices that utilize organic substances, in particular organic EL devices (organic Electro Luminescence devices), as low cost, large surface area, full color solid state display devices, is promising, and much development is being performed toward this end.
Organic EL devices are devices that cause fluorescent or phosphorescent organic molecules contained in an organic compound sandwiched between two electrodes, a cathode and an anode, to emit light in accordance with an electric current that flows in the organic compound.
This light emission is a phenomenon in which electrons are injected from the cathode, and positive holes are injected from the anode, when an electric field is applied between both of the electrodes that sandwich an organic compound light emitting layer. The electrons and the holes recombine in the light emitting layer, and energy is released as light when the energy level returns to a valence band from a conduction band.
This type of organic EL device is a device that is expected to be developed and applied to self-light emitting displays, liquid crystal monitor backlights, lighting, and the like. No matter how the organic EL devices are used, it is still desirable that the light emission maxima of the organic EL devices conform to the primary colors in order to maximize the color reproducibility and the color rendering of the organic EL devices.
Further, although it is expected that the organic EL devices will be used as backlights for liquid crystal monitors, as discussed above, three band fluorescent lamps are conventionally used as the liquid crystal monitor backlights.
That is, the liquid crystal monitors have an anti-reflection film and a supplemental light source. The supplemental light source of the liquid crystal monitors is a backlight for transmission liquid crystals, and lighting for reflective liquid crystals, while both the backlight and the lighting are employed for translucent liquid crystals.
Conventionally, a film having a low reflectance across the entire visible light region is used in the anti-reflection film, and a three band fluorescent lamp is used as the supplemental light source.
However, fluorescent lamps used in lighting and monitors (display media) such as cathode ray tubes, liquid crystal display, and the like are the main sources of light emission in the present-day office environment.
Furthermore, fluorescent lamps are generally used in liquid crystal display backlights, and it can therefore be said that nearly all of the light that enters the human eye in this environment is light from fluorescent lamps and monitors such as cathode ray tubes.
The light emission peaks of fluorescent lamps and these types of monitors are designed to conform as much as possible to the primary colors in order to maximize the color reproducibility and the color rendering of the fluorescent lamps and the monitors.
The light that enters the human eye in the present-day office environment therefore is mostly composed of the primary colors (including the vicinity of the primary colors). It is not necessary for the anti-reflection film to control reflection of all visible light regions in this type of environment, as long as the reflectance of the primary colors (including the vicinity of the primary colors) is controlled.
Considering monitor visibility, however, light emission from the fluorescent lights used as external light and light emission from the monitor backlights have identical wavelengths, and therefore a high precision is required in controlling the surface reflection of the primary color wavelengths.
There is a problem, however, with conventional anti-reflection films in that necessary and sufficient anti-reflection function cannot be obtained under the present-day office environment as described above.
Further, the organic EL devices described above are expected to be developed and applied in various areas, such as natural light displays, liquid crystal monitor backlights, and lighting, but the maximum light emission of the organic EL devices does not match the primary colors. There is a problem in that current organic EL devices are not sufficient for these applications.
In addition, the function of the organic EL devices is not sufficient for cases of using a conventional film that has a low reflectance across the entire visible light region as an anti-reflection film for a liquid crystal monitor under the present-day office environment, where nearly all of the light that enters the human eye is light from fluorescent lights and light from monitors such as cathode ray tubes. Further, mercury and fluorescent coatings are used when a three band fluorescent lamp is employed in the backlight of a liquid crystal monitor, and this is a problem because it is not ecologically desirable.